Retinoids increased the adhesive properties of cultured spontaneously-transformed mouse fibroblasts (Balb/c 3512-3 cells). Retinol, retinoic acid and their 5,6-epoxy derivatives were the most active compounds. Derivatives of retinol and retinoic acid without vitamin A activity in other systems were also inactive in increasing adhesion. Thus this system provides a relatively easy in vitro assay to test the vitamin A activity of retinoids. A new enzyme activity, retinol dehydratase, was found in these cells. The product of such enzymatic activity was identified as the hydrocarbon anhydro-retinol. Primary mouse dermal fibroblasts only possess 5% of the enzyme activity of 3T12 and 3T3 cells; however, mouse epidermal and hamster intestinal epithelial cells did not seem to contain this enzyme activity. Retinoic acid was metabolized by 3T12 cells to a compound chromatographically identical to mannosylretinyl phosphate, suggesting that a metabolite of retinoic acid may replace retinol in the phosphorylation and glycosylation pathway found for retinol in other systems.